When I submit page_styles.css to W3C I get a different message to the one you get. It says that margin-top and margin-bottom do not exist in CSS3 (but do in CSS1 and CSS2). You can use the "More Options" button in the W3C validator to select CSS2 instead.

Can Sigil be configured to use a different validator? The IDPF tool is specifically for ePub 2 and 3 documents whereas the W3C tool is for CSS in general.

But if you check the CSS 3 reference page on W3schoolshttp://www.w3schools.com/cssref/default.asp, you'll find all the usual margin properties.
Could it be that CSS 3 doesn't allow margins for paged media?
Very strange.

EPUB 3 defines a profile of CSS based on CSS 2.1 with added modules from CSS3, whereas EPUB 2 was based on a specific subset of CSS 2.

So it looks like ePub3 does not support all of CSS3. Therefore an ePub2/ePub3 validator looks more appropriate than a CSS validator. Alternatively you could use the option button on the W3C validator to use CSS 2.1.